The human psychovisual system is highly capable of appropriately adapting to illumination characteristics of an image. A human observer detects a particular range of colors for a large variety of scene illuminants. For example, the human viewer accurately perceives color of an object independent of the color of light illuminating the scene. In contrast, color digital systems such as digital cameras, frequently infer the color of the scene illuminant incorrectly.
Color appearance models can be used to model the manner the human visual system perceives the color of an object under different lighting conditions and with different backgrounds. The model is typically used so that an image viewed under one lighting condition can be adjusted to appear to have the same colors viewed under a different lighting condition.
Color appearance models are intended to attain a capability to store images in a device-independent manner. For example, a computer of an imaging system can acquire an image and a color appearance model, utilizing illumination conditions relating to the image, can transform the image into an internal representation. When the image is subsequently displayed on a display screen or printer, the color appearance can be reconstructed from the internal representation based on information relating to the output display illumination conditions to transform the internal representation to the correct representation for output display.
Color appearance models can be used to direct image processing operations to supply accurate and appropriate color reproduction, but only in a system with a suitable level of color constancy. Several color appearance models have been proposed that suitably compensate for a range of illuminants. However, these models cannot be used in digital photography and other applications since the color of the scene illumination must be known a priori. Typically this information is unavailable.
A digital camera must compensate for illuminants either by measuring the color of the scene illumination or estimate the illuminator color from the image data. In a digital imaging system, usage of an illumination sensor is impractical.
One of the problems with color appearance models is that the models are typically based on standard color or spectral definitions, such as Munsell color chips, that do not suitably cover the gamut of color and illuminants for a particular application. For example, the colors and image characteristics of digital cameras can be very different from characteristics in other areas.